Technical Field
This invention relates to a temperature control apparatus for a preform and a temperature control method for a preform, which apparatus and method control the temperature of an injection molded preform.
The present invention also relates to a resin container, and a method for producing a resin container.
Background Art
A container manufactured by blow molding a bottomed preform using polyethylene terephthalate (PET) as a resin is known. The preform is produced by injection molding, and subjected to treatment involving predetermined temperature control (see, for example, Patent Document 1). After the preform is controlled to a predetermined temperature state, it is blow-molded and stretched to obtain a container of a desired shape. The resin container manufactured by blow molding is widely used as a container which is easy to design, excellent in mass producibility, lightweight and also excellent in impact resistance.
Glass bottles are used by preference as containers accommodating cosmetics, milky lotions, etc. Containers for cosmetics are required to have an appearance, which enables the containers themselves to be worth appreciating aesthetically, in order to stimulate consumers' appetite for purchase. From this point of view, glass bottles are satisfactory in that they have a profound feel or a quality appearance, and can retain a beautiful state even after repeated use. The glass bottles, however, are heavy and tend to break, and also have the drawback of high costs involved in transportation and production.
In recent years, the use of resin containers in place of glass bottles, as containers for cosmetics, has begun to be considered. The resin containers are advantageous in that they are resistant to breakage, lightweight and easy to handle, and inexpensive. However, they pose difficulty in having an aesthetic appearance expected of glass bottles, such as a profound feel or a quality appearance. They involve the problem that they do not fit consumers' image of the cosmetics containers. Thus, a contrivance to impart an aesthetic appearance, comparable to that of glass containers, to the resin containers becomes necessary.
Glass bottles containing cosmetics, etc., for example, are formed in a large wall thickness for the emphasis of a quality appearance or a profound feel. Although the shape of the container is modified, as appropriate, in accordance with the contents, it is common practice to render the bottom of the container considerably thick-walled, and its trunk uniformly thin-walled compared with the bottom. Similarly, therefore, it is desirable that the resin container be also provided with a thick-walled bottom and a uniformly thin-walled trunk.
As a method for producing a resin container, injection molding or blow molding exists. With the injection molding method, the resulting container is generally limited to one in which the inner diameter of its mouth and the inner diameter of its trunk are equal, and the amount of resin necessary for molding is large. To produce a bottle-shaped container with the inner diameter of the mouth smaller than the inner diameter of the trunk, therefore, the blow molding method has to be employed.
The blow molding method is present in two types, a hot parison method in which a preform is not cooled to room temperature, but is blow-molded by effective use of the residual heat (internal heat quantity) during injection molding; and a cold parison method in which a preform is once cooled to room temperature, then reheated and blow-molded. For the molding of a medium- to small-sized container mainly containing a cosmetic or the like, the hot parison method, which is advantageous from the aspects of energy consumption and shapability, is usually used.
The molding of a preform is generally performed by injecting and charging a molten resin into an injection space formed by an injection cavity mold, an injection core mold, and a neck mold. The molten resin is quenched, while being solidified, down to a temperature of the order of 80° C. on its inner and outer surfaces in contact with the cooled injection cavity mold and injection core mold to form a skin layer on the surface. Since the skin layer is formed, the preform can be released from the injection mold, with its shape being retained. On the other hand, the interior of the preform has not been completely cooled yet, but is still at a high temperature of the order of 140° C.
With the hot parison method, the residual heat of each part of the preform is proportional to the wall thickness and, the larger the wall thickness, the easier the stretching of the preform becomes. Moreover, if the wall is thinned by stretching and increased in surface area thereby, the temperature of the stretched site lowers to make stretching difficult. Thus, stretching shifts to an adjacent site where the wall is relatively thick and the temperature is relatively high. Hence, a technique is adopted which, in consideration of the standard values required of a finally molded container, adjusts the wall thickness of each part of the preform, and achieves the desired wall thickness distribution in the final container.
In order to adjust the preform after injection molding to an appropriate temperature state, a temperature control device may be provided. The temperature control device is effective in mitigating an undesirable temperature distribution of the preform, which occurs during injection molding, or in locally lowering the temperature of a site to be increased in wall thickness, such as the shoulder or bottom of the container.
Even if it is attempted to impart a temperature distribution to the preform by a conventional temperature control device, however, heat is gradually transmitted to an adjacent site. Thus, it has been impossible to impart a temperature distribution which can be definitely distinguished from the temperature of the adjacent site. Hence, it has been technically extremely difficult to produce a resin container having a thick-walled bottom and a uniformly thin-walled trunk adjacent to each other. As the wall thickness is increased, moreover, uniform stretching by blow molding is more difficult, thus having posed difficulty in molding a thick-walled container free from wall thickness unevenness or distortion.
For the reasons mentioned above, a method for forming a resin container, which has a thick-walled bottom, a relatively thin-walled uniform-thickness trunk (having a satisfactory wall thickness distribution as a small- or medium-sized container accommodating a cosmetic or the like), and the feel of a glass bottle, has not been established yet.